Apparatus for chopping strand

ABSTRACT

Strands of glass filaments are advanced directly from the bushings in which they are formed to a chopper. The chopper is arranged to issue short lengths of strand along a generally horizontal trajectory to deposit the strand on a conveyor with a component of motion in the conveying direction. Cantilever mounted blade, and back-up rolls have their axes horizontal and are generally one above the other. Starting of the strand is facilitated by a back-up roll of substantially greater axial length than the blade roll so that strand can be carried over a guide and at least partially around the unobstructed, free end of the back-up roll to initiate the pulling of the strand. A holddown roll having a tapered nose and of an axial length intermediate that of the back-up roll and blade roll is brought into pressure engagement with the back-up roll after the initial strand turn is taken. The drive of the blade roll and back-up roll at the filament pull speed of the bushing is quickly achieved as the strand guide, aligned with the blade roll, directs the strand along a helical path and into the blade roll.

United States Patent [191 Genson APPARATUS FOR CHOPPING STRAND [75] Inventor: Samuel Richard Genson, Weston,

Ohio

[73] Assignee: Johns-Manville Corporation, New

York, NLY.

[22] Filed: Oct. 26, 1972 [21] Appl. No.: 300,960

Primary ExaminerWillie G. Abercrombie Attorney, Agent, or Firm-Robert M. Krone; John D. Lister June 11, 1974 57 ABSTRACT Strands of glass filaments are advanced directly from the bushings in which they are formed to a chopper. The chopper is arranged to issue short lengths of strand along a generally horizontal trajectory to deposit the strand on a conveyor with a component of motion in the conveying direction. Cantilever mounted blade, and back-up rolls have their axes horizontal and are generally one above the other. Starting of the strand is facilitated by a back-up roll of substantially greater. axial length than the blade roll so that strand can be carried over a guide and at least partially around the unobstructed, free end of the backup roll to initiate the pulling of the strand. A holddown roll having a tapered nose and of an axial length intermediate that of the back-up roll and blade roll is brought into pressure engagement with the backup roll after the initial strand turn is taken. The drive of the blade roll and back-up roll at the filament pull speed of the bushing is quickly achieved as the strand guide, aligned with the blade roll, directs the strand along a helical path and into the blade roll.

. 11 Claims, 4 Drawing Figures PATENTEDJuu 1 I I974 SHEET 2 OF 2 mm mm APPARATUS FOR CHOPPING STRAND CROSS-REFERENCE TO RELATED APPLICATION This application discloses material which is the subject of an application filed of even date in the names of JACK LAURENCE BRUNK and ERIC JOSEPH BROSCI-I entitled APPARATUS AND METHOD FOR ACCELERATING STRAND and now U.S. letters Pat. No. 3,771,70l issued Nov. 13, I973.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to apparatus for severing strands into relatively short lengths and is particularly applicable to the severing of glass fiber strands made up of gathered filaments as a process continuous with the formation of those filaments.

2. Description of the Prior Art Heretofore it has been known to sever linear materials into regular lengths by passing the material between 'a back-up roll and a blade roll mounting a series of blades spaced evenly about its periphery and extending generally along its axis. Back-up rolls have been employed as a capstan to provide the pulling force to draw filaments from their source and have been arranged with an overhang or cantilever mounting to facilitate lacing of strands into the chopper. However, where a substantial number of strands are to be chopped simultaneously, strands tend to be lost or broken during lacing of normal rolls and are laced into the chopper only with difficulty. This difficulty increases significantly where the strands are derived from a number of bushings from which their filaments must be pulled continuously during lacing.

SUMMARY OF THE INVENTION A chopper particularly adapted for direct chop applications is arranged to improve the strand handling characteristics during starting of a run by having a back-up roll having an axial length substantially greater than the axial length of the bladeroll whereby strand turns can be started on the cantilevered free end of the back-up roll. In order to enhance the frictional adhesion of the strands on the back-up roll a pressure roll is provided The effective axial length of the pressure roll is less than that of the back-up roll so that free ends of the strand can be brought into engagement with the back-up roll conveniently. That effective axial length is also longer than the blade roll so that the strands can be secured prior to the initiation of chopping. A tapered end on the pressure roll facilitates the introduction and the initial gripping of the strands. Guides aligned with the blade roll on the strand lead-in side of the back-up roll cause the strand to track along a helical path along the axial length of the back-up roll into alignment with the blade roll as the pulling of the strand is started.

DESCRIPTION OF THE DRAWING FIG. 1 is a diagrammatic front elevation of one form of the apparatus and its associated equipment for drawing continuous glass filaments from a plurality of DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1 aplurality of fiber forming bushings 11 issue filaments 12 of molten glass, in the case where strands of glass fibers are being chopped, which are drawn at speeds of the order of thousands of feet per minute over a size applicator 13 to a gathering wheel 14 which forms strands 15. These strands are guided by guide wheels 16 in parallel paths to chopper 17 having guide wheels 18. From guide wheels 18 the I strands are carried over a back-up roll 19 which can be faced with a layer of urethane 21 (FIG. 3) to enhance the frictional forces developed between the strands 15 and roll 19. Further enhancement of the filament pulling force developed at roll 19 is achieved by a pressure roll 22 which forces the strands 15 against roll 19.

Roll 19 is mounted on shaft 23 supported in pillow block 24 so that it extends as a cantilever from chopper frame 25. Pressure roll shaft 26 has bearings mounted on lever 27 to mount roll 22 in a cantilivered or overhung fashion. Strands 15 are chopped as they are carried by roll 19 into a region at the upper face' of the roll engagedby blades 28 mounted to protrude radially from blade roll 29.-Shaft 31 supports blade roll 29 as a cantilever extending from frame 25 and in turn is mounted in a pillow block 32 (FIG. 2) mounted for vertical adjustment to move shaft 31 toward and away from the shaft 23 to move blade roll 29 with respect to back-up roll 19.

- Pressure roll 22 is forced against the urethane coating 28 by a pneumatic cylinder 33 operating through a linkage 34 pivotally coupled to lever 27 pivoted on bracket 35 and supporting pressure roll shaft 26' around which the roll 22 rotates. A tapered nose 36 on the free endof pressure roll 22 guides strand into the pulling region between the right circular surfaces of the effective length of pressure roll 22 and back-up roll l9. In FIG. 3 the effective length of the pressure roll is about 5% inches while the back-up roll 19 is about 5% inch long and the blades 28 have a 2% inch long cutting edge. The tapered nose 36 in the embodiment of FIG. 4 is approximately 1 inch long while the remaining 4 inches of its length is straight and parallel to the back-up roll 19' of about 7 inches axial length and blade roll 29 having blade cutting edges about 2% inches long.

Blades 28 are of trapezoidal plan form with the sharpened edge on the minor base of the trapezoid and the major base fitting into slots in the core 37 of the blade roll 29 best seen in the sectional position of FIG. 3. Annular blade retainers 38 have inwardly directed flanges 39 with tapers matching the incline of the trapezoidal blades and are secured to the outer faces of core sources and advancing those filaments through the chopper to a collection conveyor;

37 as by machine screws 41. A blade roll cover 42 finishes each end face of the blade roll 29.

The urethanefacing 21 on back-up roll 19 wears under the blades 28. Such wear is accommodated in part by arranging for relative movement between the blade roll 28 and the back-up roll 19. Back-up roll 19 can be reversed so that its strand starter region, that region unobstructed by the blades 28, can be interchanged with its chopping region, that portion contacted by the blades. Further, the urethane facing 21 can be replaced. As shown in FIG. 3 in the sectioned portion of roll 19, the back-up roll is made up of a pair of hubs 43 which are separable from the drum 44 on which the facing 21 is secured. Hubs 43 can be fitted with either hub on the inner end of back-up roll shaft 23 and thus can be reversed by removal of the equipment outboard of the shaft and threaded collar 45 securing the outermost hub.

Adjustment between the blade roll 28 and back-up roll I9 is by movement of the blade roll relative to frame 25. Pillow block 32 for blade roll shaft 31 is mounted on a slide plate 46 guided in ways 47 for vertical motion relative to frame 25. A vernier type precision lead screw drive 48 is coupled between plate 46 and frame 25 in a manner to avoid any misalignment in the ways. Thus the blade roll position can be adjusted to compensate for wear, particularly of the urethane coating on the back-uproll, without loss of alignment of rolls l9 and 29.

Heretofore choppers have been arranged with .the blade roll positioned to cause blades to penetrate the back-up roll surface to a substantial degree, of the order of one thirty-second of an inch. While glass fibers were chopped effectively with such an orientation the resilient back-up roll surface was broken up rapidly so that new rolls were required every 8 to 12 hours. The lengthening of the back-up rolls 19 to permit interchange of its starting area with its chopping area effectively doubled the useful life of the back-up roll. However, it has been found that a further enhancement in useful life of the back-up roll surface 21 can be achieved by the precision adjustment of the relative spacing of the blade roll and back-up roll so that, when stationary, the blades only just contact the back-up roll resilient surface 21. At pulling speeds, it has been found that the urethane face 21 is displaced outward by centrifugal force sufficient to insure the chopping of the strands with a very significant reduction in the wear on that surface due to blade penetration. This roll relationship has increased the working life of the face 21 to about 72 hours with all process conditions except roll spacing corresponding to those for which the 8 to 12 hour life was experienced.

The light contact of the'blade roll with the back-up roll has been augmented with a separate positive drive for each roll. Prior choppers drive but one roll and relied on the penetrating contact to drive the other roll from the driven roll. Thus in those instances where the worm face 21 is abraded smooth and reused as a backup surface of smaller diameter, the angular speed of the shaft for the back-up roll must be increased relative to the speed of the blade roll to maintain the linear speed of the effective circumferences of the blade cutting edges and the reduced diameter back-up surface 21 constant. This individual drive with individual speed adjustment has been provided as will be described.

Strand 15 is chopped between the generally vertically aligned blade roll 29 and backup roll 19 and issues tangentially from between those rolls along a generally horizontal trajectory as chopped strand 49. This horizontal trajectory enables the strand which has advanced to the chopping station at the filament pulling speed to decelerate during its flight so that its impact on the collecting surface 51 is reduced-from the usual impact of downwardly directed chopped strand. Advantageously the trajectory from chopping station to collecting surface 51 can be about 4 feet long to provide for retardation by the drag on the chopped strand of ambient atmosphere where the pull rate and lineal surface speed of roll 19 is between about 2,600 and 4,700 feet per minute. Further reduction of impact is achieved by advancing the collecting'surface 51 with a component of motion parallel to the trajectory at its point of impact as by employing an endless conveyor belt trained over head pulley 52.

The mounting of the back-up roll 19, hold-down pressure roll 22 and blade roll 29 as cantilevers having common open ends affords access for lacing strands into the chopper on start-up. As best seen in FIGS. .3 and 4 the lacing is further facilitated by proportioning the lengths of the rolls whereby a gripping of the strands occurs before they are carried to the chopper blades. Back-up roll 19 is initially engaged by strands wrapped around its urethane face 21.

As the roll 19 is started in rotation, when initially placing the chopper in operation, the strands are placed in the guide wheels 18 which are in pulling alignment with blade roll 29. Since the strands tend to follow a straight path they initially define a helical path to the protruding end of roll 19 outboard of the pulling region between roll 19 and pressure roll 22 and are quickly drawninb'oard of roll 19 to the chopping region under blades 28.

The pressure roll 22 has a frusto conical nose 36 on its free end to enhance the ease of introduction of strands into the pulling area between its effective pressure length of its constant diameter portion and the back-up roll 19. Two embodiments of the pressure roll are disclosed in FIGS. 3 and 4. in FIG. 3'strands canbe started with the chopper running at pulling speed. An auxiliary strand starting mechanism is provided in the form of a cone 53 having a starting roll 54 bearing on its uppermost face portion so that the cone 53, which rotates with and is mounted upon the back-up roll 19 and its shaft 23, receives strand near its apex and thus at a low peripheral speed and advances that strand along the cone to the back-up roll surface 21 with which it is continuous. This embodiment of the holddown or pressure roll 22 employs a pressure roll having an effective length approaching the length of the backup roll. The frusto conical nose portion 36 has its base short of the outermost limit of the back-up roll 19 and its outermost portion overlying the starter cone 53. Thus a strand started on cone 53 is admitted to the pull region between starting roll 54 and cone 53 by the frusto conical outermost portion 55 of roll 54, then advanced at an increasing lineal speed as it climbs to the greater diameter of the inner portion of the starting cone 53 driven at the speed of rotation to provide the desired pull rate as the surface speed of back-up roll 19. The strand under the impetus of guides 18 transfers from cone 53 to roll 19 and shortly passes the nose 36 to the pull region beneath pressure roll 22. It continues its advance axially of backup roll 19 until it is aligned with its guide 18. This arrangement permits the introduction of strand to the chopper while it is running at speed.

Starting roll 54 provides means of establishing traction between the strand l5 and-the driving elements of the chopper. Since glass fibers are rather abrasive and the surface of cone 53 is relatively hard, the starting roll 54 is made flexible and readily replaceable either in whole or part by making it of a resilient material such as polyurethane or rubber in stacked disc form. The discs 56 readily conform to the cone 53 when subjected to relatively light pressure and can be replaced individually when worn.

Starting roll 54 is mounted on a bracket 57 secured to the frame 25 in vertical alignment with the back-up roll and blade roll shaft axes so that the axes of the three roll fall in a common place. Bracket 57 includes a suitable bracket 58 for the roll 54 to sustain it parallel to the face of the starting cone in condition for free rotation over that face. Telescoping elements 59 and 61 permit the bracket 58 and roll 54 to be moved toward and away from the starting cone under the influence of adjusting means 62 which can also clamp the properly positioned elements.

The elements of FIG. 4 correspond generally to those of the other figures and have been designated by the previously employed reference characters with primes added.

In the case of FIG. 4 the pressure roll 22' is shorter than the back-up roll 19 so that the extreme of its nose 36' is short of the outer extreme of back-up roll 19'. This arrangement affords an unobstructed region over which strand can be wrapped manually to initiate drawing and chopping. In using this apparatus the operator grasps the strand as in a normal fiber forming procedure as it issues from the sizing applicator l3, laces it over gathering wheel 14 and guide wheels 16 to chopper guide wheels 18' and winds the fiber approximately 180 around the unobstructed end of the back-up roll 19. He starts the motor which drives the roll 19 at this time so that as the back-up roll 19' begins to turn it carries the strand under the hold-down roll 22' and thence to the blade roll 29.

It should be recognized that since the chopper provides the drawing force for the strands and their filaments eminating from the bushing, and since the diameter of the filaments is a function of the drawing speed, the strands must be started with a gradual acceleration. In the case of the chopper of FIG. 4 a stationary or slowly rotating back-up roll is required while the starter cone 53 of FIGS. 2 and 3 provides accelerating means. The first portion of acceleration of the strand by the chopper will produce oversize filaments and strands until the chopper reaches its drawing speed.

The slow speed start of strands from a multiplicity of bushings requires the strands to be laced in sequence. In the case of FIG. 3, tolerable strand acceleration can be achieved employing the starter cone 53 while the back-up roll 19 is running at rated filament drawing speed. However, in the case of FIG. 4 a strand cannot be laced into the chopper running at rated filament drawing speed since there is no means of gradually accelerating the strand under those conditions. Accordingly, where a plurality of strands are to be laced from different bushings, the first strand can be laced and the chopper started. Succeeding strands are started by maintaining sufficient lineal speed on the surface of back-up roll 19' to avoid loss of the prior laced strands without imposing so great a speed that the wrap of the succeeding strands when taken up by the pull of backup roll 19' will disrupt either the started strands or the strand being started. When all strands are laced the 6' chopper can be brought up to speed and the chopped product collected.

Both the back-up roll 19 and blade roll 29 are driven through variable speed drives. A variable speed motor in the base (neither of which are shown) drives a V belt 63 as best seen in FIG. 4 to a pulley 64 on a main drive shaft 65 mounting a spur gear 66 engaging spur gear 67 on jack shaft 68. Jack shaft 68 drives pulley 69 for V belt 71 driving pulley 72 and blade roll shaft 31 to drive the blade roll 29 and vary its speed according to the speed of drive motor. The belt drive to pulley 72 accommodates the movement of blade roll 29 and its shaft 31. Spring loaded tensioner 73 carries an idler pulley 74 to'maintain tension in V belt 71. Main drive shaft 65 also mounts variable pitch pulley 75 driving variable speed V belt 76 trained over variable pitch pulley 77 on back-up roll shaft 23. Thus adjustment of the pitch of pulleys 75 and 77 afford adjustment of the speed of the back-up roll shaft 23 relative to main drive shaft 65 and to blade roll shaft 31. With the above arrangement the pulling speed can be adjusted and the relative speed of the back-up roll 19 and blade roll 29 can be adjusted with a single motor drive. 1

Guide wheels 18 reciprocate across the feed path of the strands to distribute wear on the blades 28 and rolls. A gear motor 78 (see' FIGS. 2 and 3) drives an eccentric 79 coupled to a line 81 pivoted on frame 25. A second link 82 parallel to link 81 extends to a tie link 83 carrying at its free end a mounting bracket 84 for two staggered guide wheel brackets 85. Rotation of the motor eccentric 79 oscillates link 81 to cause reciprocation of tie link 83 and the guide wheels in the general direction of the length of the tie link. The range of reciprocation is defined by the width of the chopping region, the length of the cutting edges of blades 28, since the inner most strand 15 guided by the inner groove of the lower guide wheel 18 should not be directed closer to the frame 25 than the inner limit of blades 28 while the outermost strand l5 guided by the outer groove of the upper guide wheel 18 should not be carried further from the frame than the spacing of the outer limit of blades 28 from the frame.

The apparatus of this invention lends itself to modifications and has utility even in those instances where certain of the features illustrated are eliminated. For example, strand can be chopped with the relative spacing of the blades 28 and back-up roll 19 such as to provide substantial penetration of surface 21 and in such instances only one of the shafts 23 and 31 need be driven positively. The spacing of shafts 23 and 31 can be fixed and no relative adjustment in driving speed of those shafts is necessary when so fixed. Various strand guide mechanisms other than those disclosed can be employed. Accordingly, it is to be understood that the above description is to be read as illustrative of the invention and not in a restrictive sense.

I claim:

1. A chopper for cutting continuous strands of fibers into short lengths including a frame, a blade roll cantilever mounted for rotation on said frame to extend one end free and to be rotatable about its cylindrical axis; a plurality of blades mounted on said blade roll extending generally parallel to the axis thereof and protruding generally radially therefrom; a back-up roll cantilever mounted for rotation on said frame to extend one end free and to be rotatable about its cylindrical axis, said back-up roll axis being parallel to said blade roll axis;

and a cylindrical working face on said back-up roll; said blade roll and said back-up roll being so spaced that said blades are engageable with said working face to define a chopping region; said working face proximate the free end of said back-up roll extending beyond the region engageable by said blades whereby strand can be engaged with said working face and free of said blades in a region of the working face proximate said blade roll and intersected by the plane defined by said blade roll and back-up roll axes at the free end of said back-up roll; and a pressure roll, said pressure roll having a cylindrical surface engaged with said back-up roll for an axial length at least including the axial length of said chopping region and short of the free end of said back-up roll whereby strand can be engaged with said working face and free of said pressure roll at the free end of said back-up roll and strand is engaged by and passes between said back-up roll and said pressure roll when said strand passes through said chopping region.

2. A chopper according to claim I wherein said working face proximate the free-end of said back-up roll extends beyond the chopping region a distance at least equal to the length of the chopping region parallel to the roll axes.

3. A chopper according to claim 1 wherein strand is advanced generally normal to the axes of said back-up roll from a first side; and said pressure roll is cantilever mounted for rotation on said frame to extend one end free and to be rotatable about its cylindrical axis.

4. A chopper according to claim 3 wherein said pressure roll has adjacent its free end a tip region of a gradually diminishing diameter contiguous with its cylindrical surface engaged with said back-up roll.

5. A chopper according to claim 1 wherein said working face is an elastomer.

6. A chopper according to claim 1 including means for adjusting the spacing between said blade roll axis and said back-up roll axis.

7. A chopper according to claim 6 wherein said adjusting means comprises a slide; ways for said slide secured to said frame and mounted to define slide motion parallel to the plane defined by the axes of said blade roll and said back-up roll and normal to said axes; and support means for one of said rolls mounted on said slide.

8. A chopper for cutting continuous strands of fibers into short lengths including a frame, a blade roll cantifor accommodating displacement of said slide along lever mounted for rotation on said frame to extend one end free and to be rotatable about its cylindrical axis; a plurality of blades mounted on said blade roll extending generally parallel to the axis thereof and protruding generally radially therefrom; a back-up roll cantilever mounted for rotation on said frame to extend one end free and to be rotatable about its cylindrical axis, said back-up roll axis being parallel to said blade roll axis; and a cylindrical working face on said back-up roll; said blade roll and said back-up roll being so spaced that said blades are engageable with said working face to define a chopping region; said working face proximate the free end of said back-up roll extending beyond the region engageable by said blades whereby strand can be engaged with said working face and free of said blades in a region of the working face proximate said blade roll and intersected by the plane defined by said blade roll and back-up roll axes at the free end of said back-up roll; and a drive; a drive train from said drive to said blade roll to drive said roll in rotation about its axis, and a drive train from said drive to said back-up roll to drive said roll in rotation about its axis whereby the drive of said blade roll is independent of the drive of said back-up roll.

9. A chopper according to claim 8 including a variable speed coupling in one of said drive trains whereby the relative rotational speed of said blade roll and said back-up roll can be adjusted.

10. A chopper according to claim 9 includes means for adjusting the spacing between said blade roll axis and said back-up roll axis.

11. A chopper according to claim 3 including a slide; ways for said slide secured to said frame and mounted to define slide motion parallel to the plane defined by the axes of said blade roll and said back-up roll and normal to said axes; support means for said blade roll mounted on said slide; means to advance and retract said slide along said ways; a drive; a first drive train from said drive to said blade roll to drive said roll in rotation about its axis; take-up means in said first drive said ways; a second drive train from said drive to said back-up roll to drive said roll in rotation about its axis; and a variable speed coupling in said second drive train to provide means to adjust the rotational speed of said back-up roll relative to the speed of said blade roll. 

1. A chopper for cutting continuous strands of fibers into short lengths including a frame, a blade roll cantilever mounted for rotation on said frame to extend one end free and to be rotatable about its cylindrical axis; a plurality of blades mounted on said blade roll extending generally parallel to the axis thereof and protruding generally radially therefrom; a back-up roll cantilever mounted for rotation on said frame to extend one end free and to be rotatable about its cylindrical axis, said back-up roll axis being parallel to said blade roll axis; and a cylindrical working face on said back-up roll; said blade roll and said back-up roll being so spaced that said blades are engageable with said working face to define a chopping region; said working face proximate the free end of said back-up roll extending beyond the region engageable by said blades whereby strand can be engaged with said working face and free of said blades in a region of the working face proximate said blade roll and intersected by the plane defined by said blade roll and backup roll axes at the free end of said back-up roll; and a pressure roll, said pressure roll having a cylindrical surface engaged with said back-up roll for an axial length at least including the axial length of said chopping region and short of the free end of said back-up roll whereby strand can be engaged with said working face and free of said pressure roll at the free end of said backup roll and strand is engaged by and passes between said back-up roll and said pressure roll when said strand passes through said chopping region.
 2. A chopper according to claim 1 wherein said working face proximate the free-end of said back-up roll extends beyond the chopping region a distance at least equal to the length of the chopping region parallel to the roll axes.
 3. A chopper according to claim 1 wherein strand is advanced generally normal to the axes of said back-up roll from a first side; and said pressure roll is cantilever mounted for rotation on said frame to extend one end free and to be rotatable about its cylindrical axis.
 4. A chopper according to claim 3 wherein said pressure roll has adjacent its free end a tip region of a gradually diminishing diameter contiguous with its cylindrical Surface engaged with said back-up roll.
 5. A chopper according to claim 1 wherein said working face is an elastomer.
 6. A chopper according to claim 1 including means for adjusting the spacing between said blade roll axis and said back-up roll axis.
 7. A chopper according to claim 6 wherein said adjusting means comprises a slide; ways for said slide secured to said frame and mounted to define slide motion parallel to the plane defined by the axes of said blade roll and said back-up roll and normal to said axes; and support means for one of said rolls mounted on said slide.
 8. A chopper for cutting continuous strands of fibers into short lengths including a frame, a blade roll cantilever mounted for rotation on said frame to extend one end free and to be rotatable about its cylindrical axis; a plurality of blades mounted on said blade roll extending generally parallel to the axis thereof and protruding generally radially therefrom; a back-up roll cantilever mounted for rotation on said frame to extend one end free and to be rotatable about its cylindrical axis, said back-up roll axis being parallel to said blade roll axis; and a cylindrical working face on said back-up roll; said blade roll and said back-up roll being so spaced that said blades are engageable with said working face to define a chopping region; said working face proximate the free end of said back-up roll extending beyond the region engageable by said blades whereby strand can be engaged with said working face and free of said blades in a region of the working face proximate said blade roll and intersected by the plane defined by said blade roll and back-up roll axes at the free end of said back-up roll; and a drive; a drive train from said drive to said blade roll to drive said roll in rotation about its axis, and a drive train from said drive to said back-up roll to drive said roll in rotation about its axis whereby the drive of said blade roll is independent of the drive of said back-up roll.
 9. A chopper according to claim 8 including a variable speed coupling in one of said drive trains whereby the relative rotational speed of said blade roll and said back-up roll can be adjusted.
 10. A chopper according to claim 9 includes means for adjusting the spacing between said blade roll axis and said back-up roll axis.
 11. A chopper according to claim 3 including a slide; ways for said slide secured to said frame and mounted to define slide motion parallel to the plane defined by the axes of said blade roll and said back-up roll and normal to said axes; support means for said blade roll mounted on said slide; means to advance and retract said slide along said ways; a drive; a first drive train from said drive to said blade roll to drive said roll in rotation about its axis; take-up means in said first drive for accommodating displacement of said slide along said ways; a second drive train from said drive to said back-up roll to drive said roll in rotation about its axis; and a variable speed coupling in said second drive train to provide means to adjust the rotational speed of said back-up roll relative to the speed of said blade roll. 